是否对常规原因装箱执行Null检查? [英] Does Performing a Null Check on a Generic Cause Boxing?

问题描述

我环顾四周，找不到答案.说我有这段代码:

I looked around for this and could not find an answer. Say I have this code:

class Command<T> : ICommand<T>
{
    public void Execute(T parameter)
    {
        var isNull = parameter == null;
        // ...
    }
}

T 可以是任何类，甚至可以是 Nullable<> .如果 T 是值类型，执行上述检查是否会引起装箱?我的理解是，这与调用 ReferenceEquals 相同，后者需要两个 object 参数，如果 T 是值类型，则这两个参数都会导致装箱，如果我理解正确的话.

T could be any class, even a Nullable<>. Does performing the check above cause boxing if T is a value type? My understanding is that this is the same as calling ReferenceEquals which takes two object arguments, either of which would cause boxing if T was a value type, if I understand correctly.

如果上述情况确实导致装箱，是否有更优选的方式来执行装箱而不导致装箱?我知道有 default(T)，但是在 int 为 0 的情况下，我正在查看此值是否为 null ，无需装箱.另外，我正在寻找一种同时满足值和引用类型的方法.

If the above does cause boxing, is there a more preferred way to do this without causing the box to occur? I know there is default(T) but in the case of int that is 0, and I am looking to see if this value is null without boxing it. Also, I am looking to do this in a way that satisfies both value and reference types.

推荐答案

否-至少在我看来不是.如果 T 是不可为空的值类型，则JIT编译器将有效地将 parameter == null 检查替换为 false .没有执行时间检查.

No - at least not in my understanding. If T is a non-nullable value type, the parameter == null check is effectively replaced with false by the JIT compiler; no execution-time check is performed.

这是只有JIT编译器才能执行的优化，因为C#编译器仅生成一种形式的代码.例如，为您的样本生成的IL为:

This is an optimization that only the JIT-compiler can perform, as the C# compiler only generates one form of code. For example, the IL generated for your sample is:

IL_0000:  ldarg.1
IL_0001:  box        !T
IL_0006:  ldnull
IL_0007:  ceq

这似乎实际上是在执行装箱操作-但我相信一个不错的JIT编译器会发现，当 T 是不可空的时， ceq 总是给出错误的结果，然后删除box操作.

That would appear to actually perform the boxing - but I trust a decent JIT compiler to spot that when T is non-nullable, the ceq will always give a false result, and remove the box operation.

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